1. Field of the Invention
This invention relates to a process for preparing multicyanate esters. More particularly, this invention relates to a continuous or semi-batch process for producing multicyanate esters.
As used herein, the term "multicyanate esters" shall mean an oligomeric mixture of esters having greater than two cyanate groups attached thereto. Illustrative of such mixtures of esters are phenol-formaldehyde resins and derivatives thereof including, but not limited to, cresol-type resins and novolac-type resins which can be substituted with at least one inert substinent, such as halogen, trihalomethyl, alkyl, alkoxy, phenyl and the like, wherein the novolac-type resins are in either the solid or liquid form.
2. Description of the Prior Art
Phenolic resins are a class of synthetic materials that have grown continuously in terms of volume and applications for over several decades. Phenolic resins are often used as bonding agents in friction materials such as brake linings, clutch facings, transmission bonds, and the like. See, e.g., U.S. Pat. Nos. 4,268,157; 4,069,108; 4,268,657; 4,218,361; 4,219,452; and 3,966,670. Phenolics are also used as molding materials, coatings, and adhesives. Phenolic resins developed for non-flammability and long-term temperature stability to 230.degree. C. have been studied in carbon-fiber composites. Potential for such composites lies in advanced aircraft applications.
While present day phenolics exhibit several beneficial properties, they also suffer from a number of disadvantages, such as thermal oxidative instability, which restricts their utility. Although the modification of phenolics with cyanato groups has obviated this disadvantage, such modified phenolics have not been commercially produced because of poor shelf life, and a gel time too short for processing using known continuous processing methods for polymers and/or chemicals. This is because smoke and volatile chemicals are generated as the phenolic cyanate ester resins are cured, and the resulting crosslinked phenolic-triazine products ("PT resins") therefrom are unstable and possess poor mechanical properties, i.e., brittle.
U.S. Pat. No. 4,831,086 discloses a new class of phenolic cyanates and a method for their production whereby a phenolic resin and a base are reacted in a cyclic ether solvent to form the corresponding salt at room temperature. The salt is then reacted with a cyanogen halide in a cyclic ether at temperatures preferably between -15.degree. and -30.degree. C. in order to increase the yield and stability of the PT resins produced therefrom by minimizing the production of side reaction impurities, such as diethylcyanamide (C.sub.2 H.sub.5).sub.2 NCN, imidocarbonate Ar--O--C(NH)--O--Ar' wherein Ar and Ar' are any aromatic radicals such as phenyl, and alkyl halide (C.sub.2 H.sub.5)X wherein X is bromine or chlorine. See also U.S. Pat. Nos. 5,124,414 and 5,137,989. However, this process is time-consuming since the reactants are combined slowly in order to minimize the presence of such impurities. In addition, in view of the fact that the reaction of salt with cyanogen halide is nearly instantaneous and highly exothermic, the reaction mass temperature increases markedly and cannot be effectively cooled by conventional cooling means. Operating at such high temperatures leads to an increase in the frequency of side reactions, the production of partially cyanated end products and the consumption of amine and cyanogen halide. By precooling the reactants to very low temperatures, i.e., below -45.degree. C., the rate of reaction, and thus the amount of exothermic energy released therefrom, can be reduced. Although the precooling reduces the amount of exothermic energy to a level which can be removed by conventional cooling means, operating as such is neither cost-effective nor energy-efficient. It is also known that as a result of the insufficient mixing of the reactants during the slow process described above, the presence of concentration and/or temperature gradients, and thus the formation of side products, increases.
It would be desirable to provide a rapid, energy-efficient process for the continuous or semi-batch production of multicyanate esters having improved stability and gel time. More specifically, it would be desirable to provide such a process which not only minimizes side reactions, but also is capable of operating at temperatures close to ambient as well as providing sufficient mixing of the reactants so as to selectively promote the primary reactions.